In their attempt to provide a reason to be moral, contractarians such as David Gauthier are concerned with situations allowing a group of agents the chance of mutual benefit, so long as at least some of them are prepared to constrain their maximising behaviour. But what justifies this constraint? Gauthier argues that it could be rational (because maximising) to intend to constrain one's behaviour, and in certain circumstances to act on this intention. The purpose of this paper is to examine the conditions under which it is rational to form, and to act on, intentions. I introduce and examine in detail what Gauthier has to say on these issues, argue that it suffers from various problems, and propose an alternative account which I claim avoids them.